A ram air turbine (RAT) is a device for generating emergency supplemental power in a wide variety of aircraft. A RAT may generate hydraulic power, electric power or both. A RAT incorporates a turbine that extracts power from an air stream proximate the aircraft in flight. U.S. Pat. No. 4,743,163 to Markunas et al., owned by the assignee of this application, and incorporated herein by reference, describes a typical RAT in current use. The turbine is coupled to suitable power generating equipment, such as a hydraulic pump for hydraulic power, an electric generator for electric power, or both in the case of a hybrid RAT.
As described in Markunas et al., the turbine includes a speed governor that changes the turbine blade position to maintain a relatively constant shaft speed to the power generating equipment. Upon assembly, it is necessary to balance the turbine rotor to minimise vibration. This involves rotating the rotor on a test stand at a relatively low speed that does not cause the blades to shift from their low speed, fine pitch position. Unfortunately, as the rotor increases in speed, the balance of the rotor shifts to some degree due to the shift in position of the turbine blades from a fine pitch to a coarse pitch and due to compression of the main governor springs.
It would be advantageous to balance the rotor at an intermediate speed, wherein the blades would have an intermediate pitch, so that the maximum imbalance would be reduced by approximately one-half. However, the power that the test stand would need to rotate the rotor at the combination of intermediate pitch and speed makes this an impractical proposition.